India's Supreme Court has temporarily banned tourism in tiger reserves. The problem? All those hotels and shops built within the protected areas for the tourists are cutting down on actual forest habitat for the endangered big cats. In addition, local governments have failed to create buffer zones of slightly less degraded forest around the parks to enable the rare tigers to thrive.

The tall trees of such a protected swath of tropical forest often rise like a wall from a denuded hillside or plain. Such stands of relatively untouched trees are meant as shelters for the rich array of plant and animal species, such as the tiger, that once spread throughout vast regions of tropical jungle. But a new scientific survey that largely agrees with the Indian court's ruling finds that the more than 16,000 independent parks and reserves around the world protected by humans from humans may not prove a long-term refuge. Interviews with scientists and other experts to assess the relative health of some 60 protected areas of tropical forest in 36 different nations reveal that roughly half are continuing to lose unique species of plants and animals.

View a slide show of some of the threats to reserves and unique species.

The problem appears to be the degradation of surrounding forestland that remains unprotected, according to the research of some 261 ecologists, biologists and other scientists published online in Nature on June 25 (Scientific American is a part of Nature Publishing Group).

The scientists surveyed their colleagues to assess the relative health of more than 30 different groups of species from apex predators, like the tiger, to vines and plants that grow on top of other plants in African, American and Asian reserves. The reserves themselves ranged in size from as little as 160 hectares to 3.6 million hectares—but size was no predictor of health.

Instead, the most important thing that determined the health of these areas was the state of the lands surrounding them. Eighty-five percent of the reserves saw the surrounding lands cleared of trees or suffering from other significant impacts. In addition, many reserves themselves saw their own jungle canopies thin out: Trees were cut down for removal and sale or to make room for hotels or even increased hunting.

As a result, the number of exotic plants and animals as well as freshwater fish declined in most reserves. But some species benefited, including trees that specialize in filling disturbed ecosystems as well as human diseases that emerge from interactions with wild animals—so-called zoonoses such as Ebola. "The fates of tropical protected areas will be determined by environmental changes both within and around the reserves," the researchers wrote in Nature, noting that, essentially, protected areas needed to be better protected. "It is not enough to confine such efforts to reserve interiors while ignoring their surrounding landscapes."

The scientists, like the Indian high court, call for more "sizable buffer zones" around such parks. They also recommend creating connections between reserve areas such as the Great Limpopo Transfrontier Park in southern Africa. Another such proposal would build a string of connected reserves down the spine of the Americas or begin to connect tiger reserves in India.

In addition, the authors also call for efforts to engage and benefit local communities—much as tiger tourism has created an industry wherever the endangered cats linger in India. Without a solution that better manages such reserves to reconcile the often competing needs of humanity and our fellow inhabitants of the planet, like the tiger, even these lands set aside as natural zoos may find themselves haunted by ghosts of species past.